The present invention relates to a color cathode ray tube having an envelope comprising a display window, a cone and a neck, a cathodoluminescent screen provided interiorly of the display window, a shadow mask adjacent to, but spaced from, the screen, a triple beam in-line electron gun system comprising at least a first low voltage focusing electrode (or electrodes) and a second high voltage focusing electrode (or electrodes) being separated from each other, and electrically conductive areas provided on the interior of the neck wall.
Such a color cathode ray tube is known from Japanese Kokai 59-228347.
In an in-line electron gun, particularly a gun in which corresponding electrodes are implemented as unitary electrodes, the various juxtaposed electrodes are held together by glass beads disposed on opposite sides of a plane containing the beam paths of the three electron beams. Viewed in cross section the glass beads can be regarded as being arranged north and south and the plane may be regarded as extending east-west. In the case of the electron gun including a bipotential focusing lens then this lens is constituted by two back-to-back arranged cup-like electrodes. The first, lower voltage electrode, generally called g3; may be at 8 kV and the second, higher voltage electrode, generally called g4, may be at 25 kV. The facing surfaces of the first and second electrodes are separated by a gap of the order of 1 mm.
Convergence drift in color cathode ray tubes having such electron guns is a well-known but not fully understood problem. This problem appears as a result of the variation in the neck potential, which variation is caused by the condition of the outside surface of the neck glass at switch-on of the high tension voltage. The initial neck potential is increased by the building-up of the neck charge due to the beam current. This building-up of the neck charge is visible as a growing misconvergence of the electron beams.
Various proposals for reducing convergence drift include increasing the size of the dam, that is, increasing in the east-west direction the extent of the electrode surface between its outer edge and the nearest aperture. In so doing the influence of the wall voltage on the lens fields is reduced. Another proposal is to reduce the size of the gap between the facing surfaces of the lens electrodes. While this will reduce the influence of the wall voltage on the lens fields it has the disadvantage that as a result of the close proximity of these electrodes stray, cold emissions are produced by the lower voltage focusing electrode. As the present day trend is to enlarge the gap to avoid the production of cold emissions, this option is not acceptable.
Japanese Kokai 59-228347 proposes eliminating convergence drift by providing metallic conductive coatings on the internal wall of the neck opposite the gap between the fifth and sixth electrodes forming the principal focusing lens of the electron gun. While such conductive coatings reduce convergence drift, they will not eliminate this problem. Additionally the production of these conductive coatings, usually as metallic mirrors, generally takes place naturally during spot-knocking when very high voltages, up to 80 kV, are applied to electrodes of the electron gun. However the extent and quality of these metallic mirrors are dependent on the activity which takes place during spot-knocking. As the level of this activity varies from tube to tube it is unpredictable and in consequence the quality and repeatability of these metallic mirrors is variable and unacceptable for volume production. Furthermore this method of producing metallic mirrors is not usable in so-called "soft-flash" cathode ray tubes because the energy available during a flash-over when spot knocking is limited due to the presence of the relatively high resistance of the internal layer provided in such tubes. Thus no conductive coatings in the form of metallic mirrors will be formed during spot knocking.
Providing metallic coatings opposite the gap between the lens electrode before the spot-knocking operating stage is not a solution because during spot knocking the metallic coatings can be damaged. Also pitting of the neck glass may occur causing loose glass particles to be deposited on the lens electrodes, which particles may comprise cold emission sources. Pitting may also lead to undesired cracking of the neck glass.